This invention relates to a method of manufacturing hollow plastic articles such as bottles, and more particularly to a method of molding hollow plastic articles by using a rotary type automatic molding machine for continuously molding hollow plastic articles (for the sake of brevity hereinafter merely termed a rotary type hollow article molding machine) of the type wherein parisons are sequentially clamped by metal mold halves, and pressurized gas is blown into the parisons for urging them to the inner walls of the metal mold halves, said method is characterized by blowing the pressurized gas against the walls of the parisons thereby forming openings through the walls adapted to blow the pressurized gas into the parisons without using a hollow needle as the prior art method.
A prior art method of manufacturing a hollow article of thermoplastic material, for example a bottle, comprises the steps of melting the plastic material, extruding the molten plastic material into a tube, usually called a parison, clamping the parison by means of a pair of metal mold halves which cooperate each other to define a bottle shaped cavity, compressing the upper and lower ends of the parison to form a sealed parison unit, piercing the wall of the parison with a hollow needle and blowing pressurized gas into the parison for expanding the same against the inner wall of the closed metal mold halves. The bottle thus molded is then cooled and removed from the mold by opening the mold halves. The blow of the pressurized gas into the parison through the hollow needle is usually performed by reciprocating the needle in the transversal direction of the parison.
The method of blowing by using the reciprocating hollow needle accompanies various disadvantages including a decrease in the production efficiency and increase in the rejects.
More particularly, although the parison is prepared by extruding molten plastic, it is already cooled and solidifies substantantially until the hollow needle is operated so that it is necessary to make sharp the operating end of the hollow needle for the purpose of attaining positive piercing. When the hollow needle having such a sharp operating end is used repeatedly, the sharp end wears-out to decrease the piercing ability. In certain cases, the hollow needle often breaks so that it is necessary to exchange the broken needle with a new one thus requiring interruption of the molding operation.
When the hollow needle is worn out or broken in this manner, insufficient piercing will be resulted because the hollow needle is reciprocated with a constant stroke. Consequently the quantity of the pressurized gas blown into the parison decreases and in an extreme case it is impossible to blow thus increasing the number of rejects. Even when the operating end of the hollow needle is sufficiently sharp, due to the elasticity of the solidified parison its side wall will be deformed radially inwardly thus preventing perfect piercing. Under such circumstances, an unsatisfactory product is obtained in which the pierced portion has been greatly deformed.
One example of such a reject is illustrated in FIG. 1 of the accompanying drawings in which metal mold halves are designated by reference numerals 1 and 1'. FIG. 1 shows a hollow needle 2 reciprocated transversely into and out of the mold but does not sufficiently pierce through the side wall of a hollow parison 3 thus failing to blow perfectly.
FIGS. 2 and 3 illustrate other examples of forming rejects. In the case shown in FIG. 3, the hollow needle 2 pushes inwardly the side wall of the not yet completely solidified parison 3 to form a protrusion 3a. However, since the diameter of the protrusion 3a is small since the thickness of the wall thereof is thin, this thin wall acts as a valve for closing the opening 3b at the operating end of the hollow needle thus preventing the flow of the pressurized gas which is supplied after the hollow needle 2 has been advanced. This may cause rupture or deformation of the hollow molded article when the mold halves 1 and 1' are opened while a high internal pressure is being applied.